As used herein, “plethysmography”, and its derivative words, refer to an externally-determined (non-invasive) measurement of a size of a body part, usually of a cross-sectional area or circumference of the body part. Also, “inductive plethysmography”, as used herein, is a plethysmographic measurement based on determination of an inductance or a mutual inductance. A “plethysmographic signal” is a signal generated by plethysmography, and usually by inductive plethysmography. The part of the body measured by plethysmography may include, singly or in combination, one or more portions of the chest, abdomen, neck, arm, or the like. Finally, a thoracocardiogram (TCG), and its derivative words, refers to a plethysmographic measurement that is at least partially responsive to cardiac size.
The inductance sensor determined may be that of a conductive loop (or loops) wrapped around a cross-section of the body part. The loop may be attached to a close-fitting garment that expands and contracts with the body cross-section. As the body cross-section expands (and contracts), the area enclosed by the loop also expands (and contracts) thereby changing the inductance of the loop. The inductance change of the loop may be converted to an electrical signal using methods known to one of skill in the electrical art.
If the loop is placed around the chest, the changes in the loop inductance may be correlated to respiration volumes. For example, U.S. Pat. No. 4,308,872 (“'872 patent”), issued Jan. 5, 1982 and titled “Method and Apparatus for Monitoring Respiration,” discloses a method and apparatus for monitoring respiration volumes by measuring variations in the patient's chest cross sectional area and is herein incorporated by reference in its entirety.
In addition to measuring respiration volumes, plethysmography may also measure cardiac volumes and aortic pulses as described in U.S. Pat. No. 5,178,151 (“'151 patent”), issued Jan. 12, 1993 and titled “System for Non-invasive Detection of Changes of Cardiac Volumes and Aortic Pulses,” and herein incorporated by reference in its entirety.
U.S. Pat. No. 6,047,203 (“'203 patent”), issued Apr. 4, 2000 and titled “Physiologic Signs Feedback System,” discloses a non-invasive physiologic signs monitoring device which includes a garment that may be worn and has a plurality of sensors disposed on the garment such that respiratory and cardiac signs may be measured and either recorded locally or transmitted remotely. The '203 patent is herein incorporated by reference in its entirety.
U.S. Pat. No. 6,551,252 B2 (“'252 patent”), issued Apr. 22, 2003 and titled “Systems and Methods for Ambulatory Monitoring of Physiological Parameters,” discloses a system and method for non-invasive, ambulatory monitoring of pulmonary and cardiac parameters. The '252 patent is herein incorporated by reference in its entirety.
The plethysmographic, or TCG, signal generated preferably by inductive measurement of a chest cross-sectional area (or circumference) will be composed of essentially three components generated from different sources. The first, and largest component of the TCG signal is caused by respiration and has a characteristic frequency that varies from about 12 breaths per minute to about 30 breaths per minute. The second, and smaller, component of the TCG signal is generated by the expansion and contraction of the heart within the chest cavity and is characterized by a frequency that varies from about 50 beats per minute to about 100 beats per minute (or more) in the resting state. The third component of the TCG signal is caused by motion or noise and cannot be characterized by an a priori range of frequencies. In order to extract cardiac parameters from the TCG signal, the cardiac component must be separated from the respiratory and noise components of the TCG signal. Although no further mention of the noise component of the TCG signal will be made, when referring to the respiratory, or pulmonary, component of the TCG signal, it should be understood to include the noise or motion component of the TCG signal as well.
Separating the cardiac component from the pulmonary component in the plethysmographic signal is difficult for two reasons. First, the cardiac and pulmonary components are composite having component frequencies close to each other (for example, 0.8-1.7 Hz cardiac frequency, 0.2-0.5 Hz pulmonary frequency). Moreover, the harmonics of the component frequencies of the respiratory signal lie directly within the cardiac component spectrum. Complete separation of the cardiac and respiratory signals, however, is not required for cardiac parameter extraction but will affect the resolution and accuracy of the extracted cardiac parameter. Furthermore, the frequencies of both the cardiac and pulmonary signals may change at different rates depending on the physical exertion of the subject. Second, the relative amplitude of the cardiac signal may be approximately 20 times smaller than the pulmonary signal and can vary by as much as a factor of three depending on the level of physical exertion thereby requiring efficient removal of the pulmonary signal in order to recover the cardiac signal.
Two methods for separating the cardiac signal from the pulmonary signal are disclosed in the '151 patent. The first method takes cardiac measurements only during breath-holding thereby eliminating the pulmonary contribution to the plethysmographic signal. Breath-holding is intrusive, however, and may cause discomfort to the subject. The second method averages the plethysmographic signal based on an external trigger signal associated with a cardiac event such as the R wave of an EKG or the upstroke of a systemic arterial pulse. The disadvantage of the average method is the loss of fine details due to the averaging.
Therefore, there remains a need for more efficient signal processing of the plethysmographic signal and extraction of the cardiac signal.
A number of references are cited herein, the entire disclosures of which are incorporated herein, in their entirety, by reference for all purposes. Further, none of these references, regardless of how characterized above, is admitted as prior to the invention of the subject matter claimed herein.